Torque converters are in particular used in motor vehicles to provide a coupling between the heat engine of the vehicle and the gearbox, which may be automatic and/or of a continuously variable type. In a torque converter, the transmission of the torque is obtained by circulation of a fluid between two bladed or finned wheels or rotors, one of which is coupled to the engine and the other to the gearbox. In their construction, these two wheels are not coupled together mechanically, so that there always exists between them the possibility of relative slipping movement. In some operating modes of the vehicle, this slipping movement is of no value and can even be detrimental to the overall performance of the drive train of the vehicle.
In addition, in parallel with the torque converter, a bridging clutch is generally provided, for the purpose of making a direct mechanical coupling between the engine and the gearbox in certain operating modes, corresponding for example to quasi stabilised speed ranges. Up to the present time, such bridging clutches have been used essentially in an "all or nothing" operating mode, that is to say they are used only in a fully engaged state or a fully disengaged state.
With a view to optimizing the performance of the drive train of the vehicle considered as a whole, it is now thought desirable to use the bridging clutch in a larger number of operating modes of the drive train, and especially, for example, during gear changing operations. In addition, the bridging clutch is tending more and more to be used, no longer only in its fully engaged and fully disengaged states, but also in a mode in which it slips, in particular with a view to damping out torque variations.
Thus, there is now an increasing tendency for the bridging clutch to be operated in a way which tends to give rise to severe heating, firstly of the contact surfaces and secondly of the oil which not only controls the clutch but also cools it.
With a view to preventing this additional heating causing accelerated wear of the liners, or causing the oil to be degraded by carbonization, it has previously been proposed to arrange on the friction liners a network of grooves which give improved circulation of the oil between the friction liner and the reaction surface. Various proposals have been made as to the form of the grooves in this type of network, all of them aiming to establish communication between the outer and inner perimeters of the annular friction liner.
However, the existence of the network of grooves gives rise to a loss of oil, even when the bridging clutch is used in its fully engaged mode and when there are no longer any differences in speed between the input shaft connected to the engine and the output shaft connected to the gearbox. In addition, the presence of these grooves in the friction liner substantially reduces the effective contact surface between the friction liner and the reaction surface, to the detriment of the torque that can be transmitted without slipping by the bridging clutch.